Method and apparatus for laying marine pipelines

ABSTRACT

A method for laying a pipeline below the surface of a body of water from a semi-submersible vessel according to which the vessel is operated in an awash condition while laying pipe in deep water, and an apparatus suited for the practice of the method.

United States Patent Broussard et al.

[ Dec. 17, 1974 METHOD AND APPARATUS FOR LAYING MARINE PIPELINES Inventors: Douglas E. Broussard; Dean P.

Hemphill, both of Houston; Robert H. Kolb, Cypress, all of Tex.; Robert H. Macy, Pascaqoula, Miss.

Assignee: Shell Oil Company, New York, NY.

Filed: Nov. 9, 1970 App]. No.: 87,667

References Cited UNITED STATES PATENTS 9/1940 Childress 61/723 X 3,214,916 11/1965 Martin 61/4 3,290,007 12/1966 Yeilding..... (ll/46.5 X 3,411,306 11/1968 Mosby 61/724 3,440,826 4/1969 Kline 61/723 3,472,035 10/1969 Broussard et a1. 61/723 3,490,406 1/1970 OReilly et a1 114/.5 D

FOREIGN PATENTS OR APPLICATIONS 630,638 6/1936 Germany 114/125 1 1,206,378 8/1959 France 61/723 Primary Examiner-Paul R. Gilliam Assistant Examiner-David H. Corbin ABSTRACT A method for laying a pipeline below the surface of a body of water from a semi-submersible vessel according to which the vessel is operated in an awash condition while laying pipe in deep water, and an apparatus suited for the practice of the method.

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J. v L l/ I 43 52 .B/"OC/JJO/O/ R fi/V/ R fif //0/6 H Macy METHOD AND APPARATUS FOR LAYING MARINE PIPELINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method and apparatus for laying marine pipeline and, more particularly, to a method and apparatus for laying such pipelines from a semi-submersible vessel.

2. Description of the Prior Art The increased development of gas and oil fields from offshore production facilities has led to an increased demand for pipe-laying apparatus capable of laying a pipeline along the bottom of the ocean for purposes such as connecting a production facility with a shore installation or with another offshore facility. Two types of vessels have heretofore been most commonly used to lay these pipelines. The first is a conventional barge suitably outfitted with pipe-laying equipment. The barge has the advantage of being a relatively shallow draft vessel which may negotiate fairly shallow water and which may be easily moved from one location to another.

The second type of vessel heretofore used to lay marine pipelines is a semi-submersible vessel. Such vessels generally have work platforms equipped for pipe-laying which are mounted above buoyancy control containers or ballast chambers which may be selectively filled with fluid to partially sink the vessel. In practice it is common to move a semi-submersible to a construction site with these buoyancy control chambers evacuated so that it is floating high in the water. At the construction site, at least some of the chambers are flooded to sink a substantial part of the vessel below the surface of the water. According to prior teaching, it is desirable to lower the semi-submersible vessel into the water until a substantial portion of the mass of the vessel is below the zone of major wave activity (usually from to 50 feet or more below the surface of the water). In this position the vessel is subject to very little wave activity and, thus, provides an extremely stable work surface.

A major disadvantage of the semi-submersible vessel is that if it is desired to sink a substantial portion of the mass of the vessel below the zone of major wave activity, it is necessary to support the work platform a considerable distance above the lowermost portion of the vessel to maintain the platform well out of the water when the vessel is in a deep draft position. When moving such a vessel from deep to shallow water, it is, of course, periodically necessary to adjust the draft of the vessel so that the lowermost portion of the semisubmersible structure does not strike the ocean bottom. This adjustment naturally raises the work platform of the vessel further above the water surface. When operating with a draft of feet or less the work platform of prior art vessels is generally at least 50 feet above the ocean surface.

Laying pipe from a substantially raised work platform, as is necessary when laying pipe in shallow waters with conventional semi-submersible vessels, presents a number of problems. In the first place, because its center of gravity is raised, the vessel becomes less stable. In the second place, it is usually desired to accomplish as much of the pipeline assemblywork as possible on a horizontal or nearly horizontal work area or deck. Commonly a'curved or inclined ramp is incorporated into the vessel to reduce bending stresses in the pipe as it leaves the vessel and enters the water. This ramp provides for support of the pipe and may commonly reach an angle of 3 to 10 degrees at the stern of the vessel. In the case of a semi-submersible or other vessel having an elevated deck, all or part of this ramp may extend beyond the vessel. If the angle of the work deck is to remain constant as the semi-submersible is raised to a shallow draft position to operate in shallow water, the distance along the projection of the ramp to the water surface increases greatly (for example, changing the elevation of the end of a ramp by 30 feet would increase the projected length of a ramp inclined at 5 degrees by approximately 350 feet). Thus, on semisubmersible vessels designed to be used according to prior art teachings, it has been required when operating in shallow water either to increase the angle of inclination or the length of the work surface assembly area or to provide a long discharge ramp to support the pipeline as it passes between the work platform of the vessel and the water surface. Vessels have been constructed which require the use of stem ramp extensions on the order of 2 to 3 hundred feet long when laying pipe in a shallow draft position. These extensions must usually remain in place during operations in the deep draft position.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method for operating a semi-submersible pipe-laying vessel when laying a marine pipeline from deep water to shallow water which will obviate the need for providing the work vessel with such an extended rampor with a steeply inclined assembly area.

The method comprises providing a work platform mounted above a semi-submersible hull at a distance sufficient to maintain the platform above waves and spray in seas of the maximum wave height in which it is desired to lay pipe with the vessel. The work platform is preferably mounted on a number of columns spaced so that when the vessel is in a deep draft position it is transparent to waves (i.e., waves may pass substantially unhindered between the semi-submerged hull and the work platform). According to the method, the platform so provided is moved to a work site in a shallow draft substantially afloat condition. Upon reaching the work site, the vessel is submerged, not so that a major portion of the mass of the vessel is below the main zone of wave activity as is taught by the prior art, but to an awash condition in which the sembsubmersible hull is maintained at or below the surface of the water but within or near the area of maximum wave activity adjacent the water surface. Pipe may then be laid from the work surface in a conventional manner. As the pipelaying vessel moves into shallow water, the draft of the vessel is adjusted as required to prevent striking the bottom. By this means, the change in work platform elevation between deep and shallow draft positions is much less than for the semi-submersibles of prior teaching.

A vessel particularly suited for the practice of this invention is one which combines the seaworthiness of a conventional barge with the rough-weather steadiness and wave transparency of a semi-submersible barge. The hull portion of this vessel is similar to a conventional barge. An elevated working platform is supported above this hull on a number of large columns which provide reserve buoyancy and stability. The vessel may be operated with the barge hull in a shallow draft condition when being towed and while working in very shallow waters or it may be ballasted down to a deep draft semi-submerged condition for operation in deeper or rougher waters. Since the vessel is to be used according to the method of this invention, the hull when in the deep draft position is merely awash so that waves pass over rather than impact against the hull. Therefore, it is necessary to support the platform above the hull only at a height that is sufficient to provide a fabrication area protected from waves and spray (usually about feet). A vessel of this design may be advantageously used for purposes other than pipe laying, for example, as a drilling barge or as a supply vessel for offshore operations. When the vessel is used for pipeline construction, a retractible or hinged bow deck extention may be provided to maintain optimum pipe assembly length while allowing the overall length of the vessel to be reduced.

A hinged ramp section may be connected at the stern of the vessel to deliver the pipe to the waters surface. This hinged unit may provide an attachment for a stinger and may be adjusted to compensate for changes in vessel draft. However, because there is a relatively small difference between the awash deep draft position and the shallow draft position when operating the vessel according to the method of this invention, the working platform of the vessel does not rise much when the vessel is lifted to the shallow draft position. Therefore, the stern ramp, if used, can be on the order of onefourth the length of stem ramps used on prior art vessels.

At times it is necessary to operate the lay vessel under conditions of high cross currents that act on the submerged hull and impose large loads on the barge anchoring or positioning systems. Large semi-submersible vessels as taught by prior art characteristically have large submerged hulls and therefore, require large anchoring or positioning systems even for nominal water currents. The smaller size of the semi-submersible of this invention therefore exhibits substantially less drag resistance to water currents for equivalent length of working stations. Moreover the vessel may be operated in the shallow draft condition, even in deep water locations reducing current drag forces still further.

At other times moderate sea conditions can generate waves of a critical length which in combination with the length of a floating vessel generate abnormally high anchoring forces. Under these conditions vessel motion may not be of any significance, in either the full floating or fully submerged conditions, since high anchoring loads due to waves are associated with short wave lengths which are of moderate height. The present design permits the optimum choice to be made with respect to minimizing anchoring loads under a variety of sea conditions. Where wave action produces the major anchoring load the hull may be submerged to minimize lateral wave forces. When currents produce the major anchoring load, the hull may be brought to the fully floating position to minimize the drag forces due to currents. In combination seas the vessel can be partially flooded to achieve minimum anchoring requirements.

The capability to minimize anchoring forces is especially advantageous for vessels designed for deepwater operation. Anchoring systems represent a substantial part of the vessel cost of a deepwater pipe-laying system. The cost increases with increased anchoring load. The present design therefore reduces anchoring forces in two ways. Inherently, the hull size is smaller than conventional vessels which in itself results in smaller anchor loads. Secondly, the operating flexibility permits a mode of operation which minimizes anchoring loads if this is of prime consideration.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view, partially in schematic form, of a pipe laying vessel adapted to be used according to the method of this invention.

FIG. 2 is an end view of the vessel of FIG. 1.

FIG. 3 is a top view of the vessel of FIG. 1 showing in schematic form pipeline assembling equipment of a type which may be used on the vessel of FIG. 1.

FIG. 4 shows a vessel such as the vessel of FIG. 1 being operated according to the method of this invention.

FIG. 5 is a cross-sectional view showing the barge hull of the vessel of FIG. l as it floats in a shallow draft position in a beam sea.

FIG. 6 is a cross-sectional view showing the barge hull of the vessel of FIG. 1 as it floats in an awash position in a beam sea.

FIG. 7 is a cross-sectional view of the barge hull of a vessel similar to the vessel of FIG. I which has been equipped with a number of ballast pens.

FIG. 8 is a top view of the barge hull of the vessel of FIG. 7.

FIG. 9 is an oblique view of a ballast pen of the vessel of FIG. 7.

DESCRIPTION OF A PREFERRED, EMBODIMENT Referring to FIG. I, we see a pipe-laying apparatus or vessel 9 particularly suited to be used in accordance with the method of this invention. The apparatus 9 comprises a single, hollow, shallow-draft hull section 10 which preferably has the shape of a conventional substantially box-shaped barge. Mounted on the barge hull 10 are a number of buoyant columns 11 adapted to support a work platform 12 above the barge hull 10. The work platform 12 may comprise a plurality of decks such as the upper deck 13 and the lower deck 14 of the figures.

The columns 11, which can be elliptical in cross sections, are preferably mounted as far as practical from the fore and aft center line 15 (FIG. 8) of the hull 10. These columns 11 may partially extend over the sides of the barge hull 10 to provide water plane area such as surfaces 16 and I7 of FIG. 2 for stabilizing the vessel 9.

The columns 11 preferably support the work platfonn 12 above the barge hull at a height no greater than that sufficient to provide a pipeline fabrication area when the vessel 9 is in awash position protected from waves and spray in the highest seas in which it is desired to lay pipe from the vessel 9, while yet providing for survival without major damage during severe weather periods in the working area. This is preferably about 1.5 times the maximum wave height in which it is desired to lay pipe. Preferably, the lowermost portion of the work platform 12 should be no more than about 25 feet above the barge hull l0.

The vessel 9 may be provided with a fixed r amp 18 along which'pipe may be moved as it is assembled to form a pipeline. The ramp 18 preferably extends from the bow 19 of the vessel to the stern 20 and is preferably inclined at an angle between 3 and 5 degrees. However, the ramp may be essentially horizontal. Near the stern 20 of the vessel, the ramp 18 may be gradually curved to conform to a large radius bend which a pipeline sliding down the ramp 18 may follow without incurring excessive bending stresses. A hinged ramp extension 21 may be operatively connected, as with a hinge means 22, to the bow of the vessel. The bow hinged ramp extension 21 is preferably adapted to pivot in a horizontal plane between a storage position in which the ramp extension 21 runs substantially parallel to the bow 19 of the vessel and a working position in which the ramp extension 21 becomes an operative part of the inclined ramp 18 (i.e., the configuration of FIG. 1). The bow ramp extension 21 provides additional work area along the assembly ramp 18 at a lower cost than lengthening the vessel 9.

A hinged stern ramp 23 may be pivotably mounted on a hinge means 24 adjacent the stem 20 of the vessel. The hinged stern ramp 23 is preferably in part supported by an adjustable support means adapted to adjust the angle at which the ramp 23 depends from the vessel 9 as the draft of the vessel changes. This support means may be tackle 25 which runs from a drive means (not shown) to a first block 26 operatively connected to a support structure such as A-frame support 27, and through a second block 28 operatively connected to the hinged stern ramp 23.

The hinged stern ramp 23 provides means for supporting a pipeline moving down the ramp l8,from the work platform 12 as it passes between the vessel 9 and the surface 29 of the body of water in which the pipeline is being laid. The hinged ramp 23 may be provided with connecting means, such as hinged connection 24a, adapted to operatively connect the ramp to a buoyant support means, such as articulated stinger 30, which provides additional support to the pipeline as it moves through the water towards the ocean bottom whether the vessel is in a shallow or deep draft position.

The work platform of the vessel is, of course, provided with apparatus suitable for the construction of a marine pipeline such as cranes, tensioners, welding units and pipe-coating equipment as well as pipe storage facilities, crew quarters, etc. (as is shown in FIG. 4). This apparatus forms no part of the present invention and is well known to those skilled in the art. Therefore, it will not be further discussed here.

The hull section of the vessel 9 is provided with means such as floodable buoyancy control or ballast chambers 50 and associated pumps 51 for selectively flooding portions of the hull 10 to adjust the buoyancy of the vessel 9. The vessel 9 may also be provided with propulsion means capable of moving the vessel in any direction according to well known dynamic positioning techniques. For example, the vessel may be provided with hydro-jet means 31 and 32 at the stern and bow 19 which are rotatably mounted so that the thrust therefrom may be directed as necessary to move the vessel frontwards, backwards or sideways.

To operate the vessel 9 according to the method of this invention, the vessel is moved to a marine pipeline construction site under its own power or by towing while in shallow draft position with the buoyancy control chambers 50 of the barge hull 10 evacuated. The bow ramp extension 21 and the hinged stern ramp 23 are preferably in the stored position (FIG. 4A). Upon reaching the marine pipeline construction site. the ramps 21 and 23 are moved into operative position and pipe laying is commenced. If the water is shallow at this point or if it is very smooth, the vessel 9 may be maintained in the shallow draft condition as the pipeline is laid on the floor of the body of water. However, for added stability in deep or rough waters, the vessel 9 may be partially submerged by flooding the buoyancy chambers 50 of the barge hull 10 with water (FIG. 4B). The buoyancy of the vessel 9 is preferably adjusted until the barge hull 10 is in an awash condition. That is, the buoyancy of the barge hull 10 is decreased until the top of the barge hull 10 is substantially at the level of the surface 29 of the body of water. Whether the barge is in the deep or shallow draft positions, the hinged stern ramp 23 is preferably adjusted vertically so that its lower end terminates near the water surface 29. This facilitates the transfer of pipe to the buoyant stinger which is generally used in laying pipe in all but the shallowest water. When in the deep draft position, the hull may be submerged as much as a few feet below the surface of the water; however, such operation tends to minimize the advantage of the method of this invention under high wave conditions.

As the pipeline and laying vessel 9 move into shallow water, the draft of the vessel may be reduced by pumping fluid from the buoyancy control chambers to in crease the buoyancy of the barge hull 10 (FIG. 4C). Because the vessel is awash when in the deep draft position, this change from deep to shallow draft will raise the work platform 10 a relatively small distance, which will vary according to vessel design, but which is preferably on the order of 5 feet. Thus, the fixed ramp 18 may be used in both deep and shallow water without subjecting a pipeline to be laid to excessive bending stresses, and without the addition of a greatly extended stern ramp when in the shallow draft position.

Operating the vessel in the semi-submerged condition in deep and rough water has the advantage of substantially eliminating all wave impact forces on the sides of the hull 10 as at 34 (FIG. 5). It also has the effect of minimizing vessel roll with wave action especially if the vessel 9 has a single barge-shaped hull 10. For example, (as illustrated in FIG. 5) a wave striking the side of a vessel having a high freeboard, such as the vessel 9 has in the shallow draft position, creates an excess of buoyancy, as represented by triangular area 36 and force arrow B, on one side of the hull l0 and a deficiency of buoyancy, as represented by triangular area 37 and force arrow F, on the other side. These two forces form a moment couple which tends to rotate the bull in a counter clockwise direction.

In contrast, when the vessel 9 is in the awash position (FIG. 6) as a wave 38 reaches the barge hull 10, a volume of water 39 moves over the hull exerting a downward force, W, on one side of the hull. On the other side a deficiency of buoyancy, represented by area 40 and force arrow F, again exists. However, in this case the moments of the forces F and W are oppositely directed and thus tend to stabilize the vessel instead of make it roll.

This stabilizing influence may be increased if the barge hull 10 is provided with a number of ballast pens 41 (FIG. 7) adapted to hold water as a wave moves across the awash barge hull 10. The pens 41 are preferably symmetrically arranged on the barge hull 10 as by having two forward and two aft, one of each pair being on each side of the centerline (FIG. 8).

The pens 41 may be provided with hinged side and end flaps, such as flaps 42 and 43 (FIGS. 7 and 9), which preferably are adapted to swing inwardly, but not outwardly. For example, these flaps may be supported by an appropriate hinge means 54 from support means such as posts 44-47 which also support rods such as rods 52 and 53 fixedly attached to the posts 4447 with appropriate connecting means, such as U- bolts, and positioned to prevent the flaps 42 and 43 from swinging outwardly. The flaps 42 and 43 preferably are positioned so that when the flaps are closed, an opening, such as slot 48 (FIG. 7), through which water may flow is left between the bottom of the flap and the barge hull 10. In operation, a wave passing a pen 41 easily washes past a hinged flap such as flap 43a (FIG. 7) and partially fills the ballast pen adding stabilizing weight to the barge hull. When the wave has moved across the hull, the water drains out through the slots, such as slot 48a beneath the flaps. The pen walls may be hinged so that when their additional stabilizing action is not required the walls may be held open to minimize the hull dimension exposed to water currents or waye action.

In summary, this invention provides an apparatus and a method for laying a pipeline below the surface of a body of water. The method comprises providing a vesselcomprising a work platform mounted on columns above a semi-submersible hull at a distance sufficient to maintain the work platform substantially above waves and spray in seas having waves of the maximum wave height in which it is desired to lay the pipeline. Thevessel may be moved in the shallow draft position to a pipeline construction site where the hull of the vessel is then submerged to an awash condition in which the top of the hull is below the surface of said water but is within area of maximum wave activity adjacent the surface, or the vessel may be operated at any intermediate condition to achieve optimum vessel performance. A pipeline may then be assembled on the work platform and laid from said work platform in a conventional manner. As the vessel moves from relatively deep water to relatively shallow water, the depth at which the hull is submerged may be adjusted to prevent striking the bottom of said body of water.

I claim as my invention:

l. A semi-submersible vessel for carrying out offshore operations comprising:

a single, hollow, water-tight barge-shaped hull member;

a plurality of open-top ballast pen means supported on the upper surface of the barge hull member for temporarily holding water as a wave moves across said barge hull;

a number of water-tight selectively floodable buoyancy control chambers within said barge hull;

pumping means in fluid communication with said buoyancy control chambers adapted to selectively flood and evacuate said chambers thereby varying the buoyancy of said vessel in said body of water;

a plurality of vertically extending buoyant stabilizing columns supported on the upper surfaces of said hull and spaced longitudinally thereon; and

work platform means supported at the upper ends of said columns for accomodating equipment for said offshore operations.

2. The vessel of claim 1 wherein said ballast pens comprise a plurality of walls for retaining water within the ballast pen and further comprise means for supporting said walls upon the upper surface of the barge hull member, said supporting means including means for supporting at least some of the walls a selected distance above the upper surface of the barge hull to allow water to drain between said so supported walls and said barge hull and out of the ballast pen.

3. The vessel of claim 2 wherein the walls of said ballast pens comprise a number of flaps; and including hinge means connecting the flaps to the support means, said hinge means allowing said flaps to swing inwardly to admit water into said ballast pens when hit by a wave; and means for preventing the flaps from swinging outwardly with respect to the ballast pen when hit by a wave.

4. The apparatus of claim 1 wherein said work platform means carries equipment for laying a pipeline on the floor of a body of water and including ramp means mounted on said work platform means running longitudinally of said vessel for supporting a pipeline to be laid by said vessel; and

hinged bow ramp extension means operatively connected to said work platform means and positioned to provide an extension for said ramp means.

5. The vessel of claim 4 wherein said columns support said work platform at a distance of less than 25 feet above the top of said barge shaped hull, but greater than the maximum wave height of the water in which the pipeline is to be laid.

6. The vessel of claim 4 including stern ramp extension means for providing a stern extension of said ramp operatively connected to the stern of said vessel with hinge means adapted to allow said stern ramp extension means to pivot about a substantially horizontal axis; and

stem ramp extension adjusting means connected to said stern ramp extension for vertically adjusting the height of at least one end of said stern ramp extension with respect to the water surface of said body of water. 

1. A semi-submersible vessel for carrying out offshore operations comprising: a single, hollow, water-tight barge-shaped hull member; a plurality of open-top ballast pen means supported on the upper surface of the barge hull member for temporarily holding water as a wave moves across said barge hull; a number of water-tight selectively floodable buoyancy control chambers within said barge hull; pumping means in fluid communication with said buoyancy control chambers adapted to selectively flood and evacuate said chambers thereby varying the buoyancy of said vessel in said body of water; a plurality of vertically extending buoyant stabilizing columns supported on the upper surfaces of said hull and spaced longitudinally thereon; and work platform means supported at the upper ends of said columns for accomodating equipment for said offshore operations.
 2. The vessel of claim 1 wherein said ballast pens comprise a plurality of walls for retaining water within the ballast pen and further comprise means for supporting said walls upon the upper surface of the barge hull member, said supporting means including means for supporting at least some of the walls a selected distance above the upper surface of the barge hull to allow water to drain between said so supported walls and said barge hull and out of the ballast pen.
 3. The vessel of claim 2 wherein the walls of said ballast pens comprise a number of flaps; and including hinge means connecting the flaps to the support means, said hinge means allowing said flaps to swing inwardly to admit water into said ballast pens when hit by a wave; and means for preventing the flaps from swinging outwardly with respect to the ballast pen when hit by a wave.
 4. The apparatus of claim 1 wherein said work platform means carries equipment for laying a pipeline on the floor of a body of water and including ramp means mounted on said work platform means running longitudinally of said vessel for supporting a pipeline to be laid by said vessel; and hinged bow ramp extension means operatively connected to said work platform means and positioned to provide an extension for said ramp means.
 5. The vessel of claim 4 wherein said columns support said work platform at a distance of less than 25 feet above the top of said barge shaped hull, but greater than the maximum wave height of the water in which the pipeline is to be laid.
 6. The vessel of claim 4 including stern ramp extension means for providing a stern extension of said ramp operatively connected to the stern of said vessel with hinge means adapted to allow said stern ramp extension means to pivot about a substantially horizontal axis; and stern ramp extension adjusting means connected to said stern ramp extension for vertically adjusting the height of at least one end of said stern ramp extension with respect to the water surface of said body of water. 